100 DECLARE EXTERNAL FUNCTION sum_proper_divisors
110 CLEAR
120 !
130 DIM f(20001)      ! sum of proper factors for each n
140 FOR i=1 TO 20000
150    LET f(i)=sum_proper_divisors(i)
160 NEXT i
170 ! look for pairs
180 FOR i=1 TO 20000
190    FOR j=i+1 TO 20000
200       IF f(i)=j AND i=f(j) THEN
210          PRINT "Amicable pair ";i;" ";j
220       END IF
230    NEXT j
240 NEXT i
250 !
260 PRINT
270 PRINT "-- found all amicable pairs"
280 END
290 !
300 ! Compute the sum of proper divisors of given number
310 !
320 EXTERNAL FUNCTION sum_proper_divisors(n)
330 !
340 IF n>1 THEN ! n must be 2 or larger
350    LET sum=1 ! start with 1
360    LET root=SQR(n)    ! note that root is an integer
370    ! check possible factors, up to sqrt
380    FOR i=2 TO root
390       IF MOD(n,i)=0 THEN
400          LET sum=sum+i     ! i is a factor
410          IF i*i<>n THEN    ! check i is not actual square root of n
420             LET sum=sum+n/i  ! so n/i will also be a factor
430          END IF
440       END IF
450    NEXT i
460 END IF
470 LET sum_proper_divisors = sum
480 END FUNCTION